Peripheral arterial occlusive disease (PAOD) patients with critical limb-threatening ischemia have improved peripheral circulation following infrainguinal revascularization. Despite this hemodynamic benefit, little change in functional tatus occurs, and many patients have residual ambulatory dysfunction. The lack of functional improvement in revascularized patients may be due to muscle wasting, myopathy, and extreme physical deconditioning secondary to their pre-existing critical limb-threatening ischemia. Therefore, the investigators hypothesize that a program of aerobic exercise training is necessary to optimize ambulation, free-living daily physical activity, and health-related quality of life through the mechanisms of improved muscle structure and function, walking economy, cardiopulmonary function, and further gains in peripheral circulation. This randomized controlled clinical trial comparing an exercise group undergoing a program of graded treadmill walking, and a delayed entry non-exercise control group will address the following two aims: (1) to determine whether a 6-month exercise rehabilitation program will improve claudication distances, free-living daily physical activity, and health-related quality of life of older, revascularized patients with PAOD, and (2) to determine whether the primary mechanisms by which exercise rehabilitation affects the above functional outcomes are through alterations in muscle structure and function, walking economy, peripheral circulation, and cardiopulmonary function. Eighty PAOD patients will be randomized into either the exercise group (n=40) or the delayed entry non-exercise control group (n=40) three to six months following successful lower extremity arterial bypass. The 6-month exercise program will consist of graded treadmill walking 3 times per week with progressive increments in exercise duration from 15 to 40 minutes, and progressive increments in exercise intensity from 50 to 80 percent of exercise capacity. The overall intent is that this study will demonstrate that exercise rehabilitation is an important adjuvant to revascularization surgery to achieve clinically meaningful gains in ambulation, free-living daily physical activity, and quality of life in PAOD patients with critical limb-threatening ischemia.